Techniques for integrating metal and metal, or metal and resin, is needed in a wide range of fields, such as the manufacture of parts used in automobiles, household electrical products, industrial machinery, and so forth, and many different adhesives have been developed for this purpose. Of these, some extremely good adhesives have been commercially available and are in use. For example, adhesives that exhibit their function at normal temperature or when heated are used to bond and integrate metals with synthetic resins, and this method is the standard bonding method in use today.
Meanwhile, bonding methods that do not involve the use of an adhesive have been researched. An example is a method in which a high-strength engineering plastic is integrated with a light metal such as magnesium, aluminum, or an alloy of these, or an iron alloy such as stainless steel, without any adhesive being interposed between the materials. For instance, as a method for simultaneously bonding by injection or another such method (hereinafter referred to as “injection bonding”), a manufacturing technique has been developed in which a polybutylene terephthalate resin (hereinafter referred to as PBT) or a polyphenylene sulfide resin (hereinafter referred to as PPS) is injection bonded to an aluminum alloy (see Patent Documents 1 and 2, for example). In addition, it has been proven in the past that magnesium alloys, copper alloys, titanium alloys, stainless steel, and the like can be injection bonded by using a similar type of resin (Patent Documents 3, 4, 5 and 6).
These inventions were all made by the present inventors, but they are based on simple bonding theory. These are the “NMT” theory related to the injection bonding of aluminum alloys, and the “new NMT” theory related to the injection bonding of all metal alloys. One of the present inventors, Ando, who is the creator of the “new NMT” theory that can be used in a broader sense, has described the theory as follows. To produce injection bonding and its tremendous bonding strength, various conditions pertain to both the metal alloy side and the injected resin side, and starting with the metal side, the following three conditions have to be met. Condition (1) is that the metal alloy have a rough profile curve (roughness curve) in which chemical etching produces a period (spacing) between peaks or between valleys of 1 to 10 μm, and the peak-valley height difference is preferably about one-half this, specifically, about 0.5 to 5 μm.
Obtaining a rough surface such as this 100% by chemical reaction is actually impossible; more specifically, condition (1) is deemed to have been satisfied if a roughness curve can be plotted in which the texturing appears at an irregular period between 0.2 and 20 μm and the maximum height difference thereof is between 0.2 and 5 μm, or if scanning analysis by scanning probe microscope reveals a rough surface (roughness) in which the mean width of the profile elements (RSm) of the profile curve elements referred to in JIS standards (JIS B 0601:2001) is 0.8 to 10 μm and the maximum height of the maximum height of profile (maximum height roughness) (Rz) is 0.2 to 5 μm. The present inventors call this “a surface with micron-order roughness” for short. There is also a condition (2), which is that there be the above-mentioned large textured surface, or more precisely, a finely textured surface with a period of at least 10 nm, and preferably 50 nm, on the inner walls of the depressions. The last one is condition (3), which is that the surface that forms the fine texturing have ceramic layer, or more specifically, a metal oxide layer that is thicker than the natural oxidation layer, or an intentionally produced metal phosphorus oxide layer. It is also preferable if this hard layer is a thin layer with a thickness of only a few nanometers to a few dozen nanometers.
The condition on the resin side is that it be a hard crystalline resin, which can be compounded with another suitable polymer, for example, to slow down the crystallization during quenching. Actually, a resin composition in which another suitable polymer and glass fibers have been compounded with PBT, PPS, or another such crystalline resin can be used. These can be used to perform injection bonding in an injection molding mold and a standard injection molding machine; this process is described according to the “new NMT” theory of the inventors. The injected molten resin is guided into a mold whose temperature is about 150° C. than the melting point of the resin, but it is seen to be cooled in the runner and other channels and drop below its melting point. Specifically, it will probably be understood that even though the temperature drops below the melting point when a molten crystalline resin is quenched, crystals of that resin are produced and the resin changes into a solid in zero time.
In other words, a state in which the resin is molten while being under its melting point, which is called a super-cooled state, only exists for an extremely short time. With PBT or PPS that has been specially compounded as discussed above, this super-cooling time is thought to be slightly longer, and this was utilized so that the resin would penetrate into the large depressions on the micron-order metal before its viscosity was sharply increased by the production of a large quantity of microcrystals. The molten resin continues to cool even after penetrating these depressions, and the number of microcrystals increases and the viscosity rises sharply. Because of this, whether or not the molten resin can reach the deepest part of the depressions is determined by the size and shape of the depressions. Experiment results indicate that regardless of the type of metal, the resin penetrated quite far as long as the depressions had a diameter of at least 1 μm and a depth of 0.5 to 5 μm. Furthermore, if the inner walls of the depressions are rough when viewed microscopically, part of the resin will also penetrate into the gaps of this ultrafine texturing, and as a result, even if a pull-out force is applied to the resin side, the resin will hang on and be resistant to coming loose.
If this rough surface is a metal oxide, it will be hard and have a hooking effect much like a spike. If the texturing period is at least 10 μm, the result will be weaker bonding, but the reason for this is clear. Specifically, if we consider a cluster of dimple-like depressions as an example, the larger is the depression diameter, the fewer dimples there will be per unit of surface area, and as the depressions become larger, the above-mentioned spike (hook) latching effect is diminished. As to the bonding itself, it is a question of the resin component and the metal alloy surface, but when reinforcing fiber or an inorganic filler is added to a resin composition, the coefficient of linear expansion of the resin as a whole draws closer to that of a metal alloy, so it is easier to maintain bonding strength after bonding. According to this hypothesis, when a PBT or PPS resin or the like is injection bonded to the surface of a magnesium alloy, copper alloy, titanium alloy, stainless steel, or the like, the result is a strong integrated material with a shear breaking force of 200 to 300 Kgf/cm2 (approximately 20 to 30 N/mm2, or 20 to 30 MPa).
The present inventors proved the “new NMT” theory to be true by injection bonding many different metal alloys, but the hypothesis used here is based on an assumption related to a fundamental portion of polymer physical chemistry, and ordinarily would have to be reviewed by many chemists and scientists. For instance, the inventors have taken it upon themselves to discuss molten crystalline resin during quenching, but as to whether or not the crystallization rate really does drop, this was not something that was debated in the past from the perspective of polymer physics, and while it is believed to be true, frankly it has not yet been proven. Specifically, this is a fast reaction that takes place under high temperature and pressure, making direct measurement impossible. Also, this hypothesis sets forth a completely physical anchor effect theory for bonding, and is not in complete agreement with conventional wisdom and standard theory. Specifically, most of the current books written by specialists in the field of adhesion ascribe this to chemical processes.
The present inventors resigned themselves to the difficulty of direct experimentation that would lead to a proof of their hypothesis, they decided to take an opposite approach. Specifically, seeing that the “new NMT” theory can also be applied to adhesive bonding, they determined to corroborate high-performance adhesion by a similar theory. Namely, they used a commercially available multi-purpose epoxy adhesive, varied only the surface condition of the adherend, and sought to find a bonding system that was heretofore unknown.
As to bonding with an adhesive agent, there has already been wonderful progress, and this sophisticated technology has been put to use in the assembly of aircraft. This technology involves a surface treatment that imparts corrosion resistance and minute texturing to a metal alloy, and the use of a high-performance adhesive. However, when it is examined more closely, the surface treatment of the metal seems to be treatment methods that were developed over 40 years ago, such as phosphating, chromating, and anodizing, and even today these methods are used as standard procedure, so progress seems to have come to a halt. Meanwhile, as to the development of the adhesives themselves, mass production of instant adhesives began decades ago, and ever since the much-touted debut of second-generation acrylic adhesives, there has been no word of anything revolutionary.
As to adhesion theory, although the most recent scholarly trends are not known to the present inventors, commercially available books are a vague mix of chemical theory and physical theory, making it seem unlikely that any significant progress will be made in materials. The present inventors were fortunate enough to be working in an era in which the electron microscope, which has a resolution down to just a few nanometers, can be freely and inexpensively used, and looking at these high-resolution micrographs made it possible to come up with the hypotheses related to “NMT” and “new NMT” injection bonding. As a result, they arrived at the above-mentioned hypothesis based entirely on an anchor effect. Consequently, it was anticipated that some new discovery would be made if the physical aspect were given emphasis in experiments into adhesion theory by adhesive bonding.
Meanwhile, copper and copper alloys have the best electrical and thermal conductivity of all practical metals, and also have excellent corrosion resistance. Their specific gravity is around 8.9, and while this makes them relatively heavy metals, they are used in a vast range of applications because of their above-mentioned performance. The present inventors have begun trial production of relay case take-off terminals from tough pitch copper C1100 copper alloy rod and PPS resin using an injection bonding method that has already been developed (Patent Document 4), and wondered if heat diffusers for mobile electronic devices and the like, lead wire take-offs for anti-explosive devices, and other such parts could be manufactured by using an adhesive agent, rather than by injection bonding. In particular, when it comes to tensile strength, carbon fiber reinforced plastic (hereinafter referred to as CFRP) is one of the best of all structural materials, including metals, and it is also super-light, with a specific gravity of 1.6 to 1.7. The inventors thought that parts that take advantage of both light weight and the advantages of copper could be produced by combining this CFRP with a copper alloy having a higher specific gravity.
A CFRP prepreg is a weave or cluster of carbon fiber (hereinafter referred to as CF) that has been impregnated with uncured epoxy resin, and simultaneous curing is possible, and integration is easy, if there is good compatibility with the epoxy adhesive applied to the metal side. Therefore, in producing an integrated product, the inventors felt that the first focus of research and development should be how high the bonding strength between a copper alloy and an epoxy adhesive could be increased and how stable it could be made. A copper alloy also exhibits good corrosion resistance even in seawater with a high salt content. And not only is corrosion resistance good, but very little seaweed adheres to copper parts in seawater and fresh water. For example, it is known that if bacteria adhere to copper or silver coins, they are killed, and this effect is attributed to the redox capability of the tiny amounts of copper ions and silver ions that are dissolved out. To put it another way, these metals are used as coins because of this effect, and it is surmised that this is also the reason why almost no seaweed adheres to the copper plates attached to ships hulls.
It is common knowledge among seafarers that a ship with aged hull paint will become completely covered in seaweed upon mooring in port for just a few days in the summertime. Hull paint itself releases ions of copper or tin in very small amounts at a time, and although there have been improvements of late, it is still a vivid memory that seawater fouling occurred with past hull paint with good anti-seaweed performance. It can be readily understood that no anti-seaweed paint would be necessary if an FRP ship covered with thin copper plates could be manufactured. For example, FIG. 4 shows the tip of a seaplane pontoon made of CFRP covered with a thin copper alloy plate, which is an idea of the present inventors. It is not known whether or not such ideas or specific challenges existed in the past, but if a copper plating that could be adhesively bonded to FRP at extremely high strength could be obtained, it would not be difficult to product a practical pontoon having such a structure.
Because of the above, an attempt was made to develop a method for obtaining a strong bond with a fiber reinforced plastic (hereinafter referred to as FRP), focusing on the development of technology for the surface treatment copper alloys.    Patent Document 1: WO 03/064150 A1    Patent Document 2: WO 2004/041532 A1    Patent Document 3: PCT/JP2007/073526    Patent Document 4: PCT/JP2007/070205    Patent Document 5: PCT/JP2007/074749    Patent Document 6: PCT/JP2007/075287